1. Field of the Invention
The present invention relates to a wavelength conversion apparatus and a two-dimensional image display apparatus, and more specifically to a wavelength conversion apparatus capable of stably providing high output harmonic laser light by a combination of a fiber laser and a wavelength conversion element, and a two-dimensional image display apparatus using the same.
2. Description of the Background Art
A visible-light source providing a high, strongly monochromatic output of class W is necessary to realize large displays, high luminance displays and the like. For a red light source among RGB (red, green and blue) light sources, a red high output semiconductor laser used in DVD recorders and the like is usable as a highly productive compact light source. However, it is difficult to realize green and blue light sources with a semiconductor laser. A highly productive compact light source is demanded for these colors.
For a green or blue light source, a wavelength conversion apparatus obtained by combining a fiber laser and a wavelength conversion element is conventionally realized as a low output visible-light source. Such a wavelength conversion apparatus uses a semiconductor laser as a light source of excitation light for exciting a fiber laser and uses a nonlinear optical crystal as a wavelength conversion element.
FIG. 10 schematically shows a structure of a conventional wavelength conversion apparatus. In this example, green output light is obtained using the wavelength conversion apparatus shown in FIG. 10. As shown in FIG. 10, the conventional wavelength conversion apparatus includes a fiber laser 40 for outputting a fundamental wave, a wavelength conversion element 41 for converting the fundamental wave to green laser light, and a lens 42 for collecting the fundamental wave on an end surface of the wavelength conversion element 41.
First, a basic operation of the fiber laser 40 will be described. Excitation light (excitation laser light) output from an excitation laser light source 43 is incident on a fiber 44 from an end 44 thereof. The excitation light incident on the fiber 44 is absorbed by a laser activating substance contained in the fiber 44 and then converted to seed light of the fundamental wave inside the fiber 44. The seed light of the fundamental wave is repeatedly reciprocated in a laser cavity. The laser cavity includes a fiber grating 44b formed in the fiber 44 and a fiber grating 45b formed in a fiber 45 as a pair of reflecting mirrors, and the seed light is reflected by, and reciprocated between, the fiber grating 44b and the fiber grating 45b. Concurrently, the seed light of the fundamental wave is amplified by a gain provided by the laser activating substance contained in the fiber 44. Thus, the light intensity is increased and also wavelength selection is performed. As a result, laser oscillation occurs. The fiber 44 and the fiber 45 are connected to each other at a connection section 46. The excitation laser light source 43 is current-driven by an excitation laser light current source 47.
The fundamental wave which is output from the fiber laser 40 is incident on the wavelength conversion element 41 via the lens 42, and is converted into a harmonic by a nonlinear optical effect of the wavelength conversion element 41. The obtained harmonic is partially reflected by a beam splitter 48, but the rest of the harmonic is transmitted through the beam splitter 48 and becomes green laser light. This green laser light is the output light from the wavelength conversion apparatus.
The part of the harmonic reflected by the beam splitter 48 is received by a receiving element 49 for monitoring the output light from the wavelength conversion apparatus and converted into an electric signal to be used. An output control section 50 controls the excitation laser light current source 47 such that the electric signal obtained by the receiving element 49 has a desired strength, and thus adjusts the driving current of the excitation laser light source 43. In this manner, the conventional wavelength conversion apparatus adjusts the intensity of the excitation light which is output from the excitation laser light source 43 and also adjusts the intensity of the fundamental wave which is output from the fiber laser 40, and thus can provide stable output light.
As another light source, for example, Japanese Laid-Open Patent Publication No. 2006-19603 (hereinafter, referred to as “patent document 1”) proposes a wavelength conversion apparatus capable of stably providing output light by fixing the wavelength of the fundamental wave. FIG. 11 schematically shows a structure of the conventional wavelength conversion apparatus described in patent document 1. Referring to FIG. 11, a reflecting film is provided on one end surface of a laser medium 51, and a reflection preventing film is provided on an outgoing end of the laser medium 51. A fundamental wave which is output from the laser medium 51 is collected inside a wavelength conversion element 53 by a lens 56, and a part of the collected fundamental wave is wavelength-converted and output as a harmonic. The fundamental wave and the harmonic output from the wavelength conversion element 53 are collected on a surface of a wavelength selection mirror 55 by a lens 57. The wavelength selection mirror 55 reflects the fundamental wave and transmits the harmonic. The fundamental wave selectively reflected by the wavelength selection mirror 55 is fed back to the laser medium 51 via the opposite path. In this manner, the oscillation wavelength of the laser medium 51 can be fixed to the wavelength of the fed-back light. Namely, the conventional wavelength conversion apparatus can automatically fix the oscillation wavelength of the laser medium 51 to the phase-matching wavelength of the wavelength conversion element 53 and thus can provide stable output light.
The conventional wavelength conversion apparatuses shown in FIG. 10 and FIG. 11 are capable of stably providing relatively low output harmonic laser light, but have a problem of not capable of easily providing high output harmonic laser light of class W.
In addition, the conventional wavelength conversion apparatuses shown in FIG. 10 and FIG. 11 occasionally cause the following phenomenon. When LiNbO3 or LiTaO3 with a polarized inversion structure is used for the wavelength conversion element 41 or 53, a third harmonic is generated in addition to a second harmonic due to the large nonlinear optical constant of the wavelength conversion element 41 or 53, and the third harmonic causes the second harmonic to be absorbed. Therefore, in the case where a fundamental wave of a certain power density and a second harmonic both exist, the temperature rises on and in the vicinity of an outgoing surface of the wavelength conversion element 41 or 53. This causes a problem that the phase-matching condition is destroyed (i.e., thermal dephasing occurs) and as a result, the light emitting efficiency is lowered.